1. Field of the Invention
The present invention relates to a sheet stacking apparatus configured to stack sheets with improved alignment in a sheet discharging direction and, more particularly to a sheet stacking apparatus configured to stack a large number of sheets effectively utilizing a space, and an image forming apparatus having such a sheet stacking apparatus.
2. Description of the Related Art
In recent years, advancement in image forming techniques has made image forming speed faster and an image forming apparatus configured to form an image on a sheet has become capable of discharging sheets from its main body in larger quantities at higher speed. Accordingly, a sheet stacking apparatus which is connected to the main body of the image forming apparatus and stacks sheets discharged from the apparatus main body is required to be capable of stacking a large number of sheets while maintaining precise stack alignment. A sheet stacking apparatus meeting such requests (hereinafter referred to as “stacker”) is discussed in Japanese Patent Application Laid-Open No. 2006-124052.
This conventional stacker is shown in FIG. 16. The stacker 500 receives a sheet discharged from a main body of an image forming apparatus at an inlet roller 501 and then passes the sheet to a gripper 503 using a conveyance roller pair 502. The gripper 503 grips and conveys the sheet so that the leading edge of the sheet abuts against a leading edge stopper 504. At the leading edge stopper 504, the sheet is released from the gripper 503 and falls onto a stacker tray 505. At this time, the sheet falls between the leading edge stopper 504 and a trailing edge stopper 508 so that the leading edge and the trailing edge of the sheet on the stacker tray 505 are aligned. Further, if necessary, the sheet is aligned in the width direction (i.e., a perpendicular direction to the sheet conveyance) by a width alignment device (not shown) in order to align the end (side end) of the sheet. Furthermore, the sheets are pressed against the stacker tray 505 by a leading edge pressing member 506 and a trailing edge pressing member 507 in every predetermined number so that the stacked sheets do not interfere with the subsequent discharged sheet.
However, in the conventional stacker 500, a stacking space on the stacker tray 505 is adjusted to a maximum size of the sheet. Thus, in a case when a sheet of a smaller size is stacked, an area X shown in a broken line in FIG. 16 is left unused. Consequently, a dead space is made in the conventional stacker 500.
Further, a distance between the leading edge stopper 504 and the trailing edge stopper 508 of the conventional stacker 500 is set to be a few millimeters longer than the length of the sheet in the sheet conveying (discharging) direction so that the sheet easily falls between the stoppers. Accordingly, Sheet alignment of the conventional stacker 500 is inadequate since the sheet is stacked with a variation of a few millimeters between the stoppers.
However, if the aforementioned distance is precisely set to meet the length of the sheet to improve alignment, the sheet takes longer time to fall between the two stoppers due to contact with them.
Thus, in order to reduce falling time, the conventional stacker 500 presses the sheet against the stacker tray 505 with a hitting member.
However, the conventional stacker 500 has a problem that at least one of the leading edge and the trailing edge of the sheet rubs against the stoppers, thereby damages the sheet while being stacked on the stacker tray 505.
Also, it is possible that the sheet damaged by the stacker needs to re-form the image on the sheet, therefore, the productivity of the image forming apparatus having such a stacker has been poor.